1. Field of the Invention
This invention relates to an apparatus and a method for the manufacture of railway rails whereby improvements of rail physical properties and rates of manufacturing are achieved.
2. Description of Prior Art
Work conducted by various investigators throughout the 1970's and into the 1980's has demonstrated that steel railroad rails with a metallurgical structure composed of very finely spaced pearlite or a combination of very fine pearlite with a small volume fraction of bainite (sometimes referred to as transitional pearlite) give the best combination of physical properties (strength, hardness, toughness and wear resistance). See, for example, Smith, Y. E. and Fletcher, F. B., "Alloy Steels for High-Strength, As Rolled Rails", Rail Steels--Developments, Processing, and Use, ASTM STP 644, D. H. Stone and G. C. Knupp, Eds., American Society for Testing Materials, 1978, pp. 212-232; Heller, W. and Schweitzer, R., Railway Gazette International, October 1980, pp. 855-857; and Tamura, Y. et. al., "Development of the Heat Treatment of Rails, Nippon Kokan Technical Report, Overseas No. 29 (1980) pp. 10-20.
The inventors are aware of two methods currently in production to achieve these metallurgical structures, as described below.
(i) Method one involves reheating the rolled rail section from room temperature to a temperature above the ferrite to austenite transformation temperature and rapidly cooling the rail at a predetermined cooling rate. Tamura, et al. mentioned above, and Hollworth, B. R. and R. K. Steele, "Feasibility Study of On Site Flame Hardening of Rail", American Society of Mechanical Engineers, 78-RT-8, teach different approaches to this art and both are successful in achieving the finely spaced pearlitic structure desired.
(ii) The second method involves alloying the standard carbon-manganese rail steels with elements such as chromium, molybdenum or higher levels of manganese, either singly or in various combinations, such that the metallurgical changes that take place during natural cooling after the hot rolling process result in the fine pearlitic structures desired. These types of rail steel may be further alloyed with such elements as silicon, vanadium, titanium and aluminum, either singularly or in various combinations to further improve properties by various mechanisms known to those skilled in the art of rail steel metallurgy.
The heat treatment method described above has the disadvantages of the costs of reheating, handling and time involved in the separate manufacturing process and all systems in commerical operation suffer from low productivity rates. The alloy method, while avoiding the disadvantages of the heat treatment method, is costly due to the requirements for expensive alloy additions.
It has been the dream of rail mill metallurgists since the early 1900's to achieve improved rail properties by the accelerated cooling of the rail as it leaves the hot rolling mill and various publications and patents have taught art concerning this approach. See, for example, Absalon, B. and Feszczenko-Czopiwski, J., "Production of Hardened Rails", Third International Meeting on Rails, Budapest 8-12.9.1935, Hungarian Association for Testing Materials, Budapest, 1936; Canadian Pat. No. 1,024,422, "Method of Treating Steel Rail", Bethlehem Steel Corporation (Robert J. Henry), Jan. 17, 1978; and Canadian Pat. No. 1,058,492, "Process for Heat Treatment of Steel", Fried. Krupp Huttenwerke A.G. (Wilhelm Heller), July 17, 1979.
All early attempts at this approach, hereinafter referred to as "in-line heat treatment", failed to achieve a viable commercial manufacturing method due to the inability to consistently control the operation. Most of these methods were aimed at achieving preselected cooling rates such that the hot steel rail cooled to or near to room temperature with the cooling rate fixed at about 6.degree. to 9.degree. F./second in the temperature range of approximately 1400.degree. to 1100.degree. F.
It has been proposed to achieve the desired cooling rates using compressed air, steam, hot water and water modified with polymers. For example, Absalon et al., and Canadian Pat. No. 1,024,422, mentioned above, refer to the use of steam and hot water.
The direct use of unheated water has resulted in over-cooling the surface region of the rail, causing the formation of martensite. Each of these controlled cooling rate methods offers its own advantages but a common disadvantage is the difficulty of maintaining the necessary constant conditions in the production facilities required to achieve the critical cooling rates. Indeed, the variation in temperature from rail to rail plus the variations in temperature along the length of the rail as it leaves the hot rolling mill cause the temperature at the start of the cooling process to vary as much as .+-.100.degree. F. from the aim starting point. This fact alone means that no suggested constant cooling rate process known to the applicants, can be applied to conventional rail mills presently in operation.
In some approaches, attempts were made at achieving a more wear resistant rail by quickly cooling the rail surface directly after rolling to a temperature below the martensite start temperature and then allowing the core heat to soak back to the surface to temper the martensite. The resultant metallurgical structure is called sorbite (self-tempered martensite is also a term commonly used) and is the object of the Neuves-Maison method and variations of it referred to by Absolon et. al. Although this approach was successful in achieving a hard, wear resistant surface, the shell of sorbite over a core of pearlite resulted in metal fatigue at the sorbite-pearlite interface due to the abrupt change in material hardness. This fatigue becomes critical with heavily loaded wheels on modern trains and results in sudden, catastrophic rail failure. Modern rail steel metallurgists recognize the need to have a graded metallurgical structure such that there are no sudden changes in material hardness (see, for example, Nippon Kokan Technical Report, Overseas, N29(1980) referred to above).